Show that $\Phi(\phi)=e^{i m_{r} \phi}=\Phi(\phi+2 \pi)$ (that is, show that $\Phi(\phi)$ is periodic with period 2$\pi$ if and only if $m_{l}$ is restricted to the values $0, \pm 1, \pm 2, \ldots$ (Hint: Euler's formula states that $e^{i \phi}=\cos \phi+i \sin \phi . )$